Leaner than desired air-fuel ratios in an engine may be caused by unmetered air entering the engine via leaks in an engine intake manifold. For example, a leaky canister purge valve may allow additional air into the engine intake manifold. Alternatively, a degraded mass air flow sensor may also result in leaner than desired engine conditions. Engine conditions with leaner than desired air-fuel ratios can degrade engine performance and increase emissions. Accordingly, various approaches may be employed to diagnose reasons for lean engine conditions.
Example diagnostic methods may include detecting degradation in the mass air flow sensor, exhaust gas sensor, and/or leaks in the canister purge valve. Another example diagnostic approach is shown by Schnaibel et al. in U.S. Pat. No. 6,886,399 wherein intake manifold pressure is monitored to determine a leak in the intake manifold. Specifically, intake manifold pressure is monitored after engine shut down and after an intake throttle is closed. Further, air flow into the intake manifold from other sources such as an exhaust gas recirculation (EGR) valve and a canister purge valve is also terminated while monitoring changes in the intake manifold pressure after engine shut down. If intake manifold pressure increases at a rate higher than a pre-determined threshold rate, a leak may be indicated.
The inventors herein have recognized potential issues with the example approach in U.S. Pat. No. 6,886,399. As one example, the rate of increase in intake manifold pressure after engine shut down may be different based on a position of each intake valve and/or exhaust valve of each cylinder. For example, if the engine shuts down with intake and exhaust valves of multiple cylinders in an open position, the cylinders and the intake manifold may be exposed to the atmosphere via the exhaust passage. Herein, the rate of change in intake manifold pressure may be substantially different relative to the rate of change in intake manifold pressure when fewer cylinders are open to the atmosphere. As such, these differences in rate of change of intake manifold pressure may cause errors in the diagnosis of leaks in the intake manifold. To reduce such errors, a controller of the engine may be programmed with look-up tables indicating an expected rate of change of intake manifold based on various positions of each intake valve and exhaust valve of each cylinder. Herein, the leakage diagnosis may be more complicated, more time consuming as well as having reduced efficiency.
In one example, the issues described above may be at least partly addressed by a method for an engine, comprising adjusting all intake valves closed in each cylinder of the engine responsive to vacuum in an intake manifold reaching a pre-determined vacuum during engine shut down, and indicating a leak in the intake manifold based on a change in a level of vacuum in the intake manifold. In this way, leaks in the intake manifold may be detected in a more reliable manner.
As an example, a leak check for an intake manifold in an engine may be initiated during an anticipated engine shut down. As the engine spins down to rest, air flow into the intake manifold may be discontinued by closing an intake throttle as well as other supplementary air flows including exhaust gas recirculation, canister purge, etc. Piston motion in the cylinders of the engine may generate vacuum in the intake manifold. The vacuum in the intake manifold may be monitored and once a pre-determined vacuum level is reached, all intake valves of all cylinders of the engine may be adjusted closed. For example, electro-mechanical actuators may be utilized to close all the intake valves. In another example, all exhaust valves of all cylinders of the engine may be adjusted closed. A change in the vacuum level in the intake manifold after closing all intake valves may indicate a leak. Specifically, a decrease in the vacuum level in the intake manifold may indicate a leaky intake manifold.
In this way, leaks in the intake manifold may be determined in a simpler manner with higher accuracy. By closing all intake valves (or all exhaust valves) of all cylinders of the engine, the intake manifold may not be exposed to the atmosphere and a desired level of vacuum may be trapped in the intake manifold each time the test is initiated. Further, by trapping the same desired level of vacuum in the intake manifold at each leak test, more reliable results may be obtained. Further still, the leak test may be performed without relying on look-up tables for different rates of change in intake manifold pressure. Overall, the leak test may be less complex and may be performed more efficiently.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.